1. Field of the Invention
The present invention relates to a novel method of preparation of foamed articles of polyvinyl chloride resins. More particularly, the present invention relates to a novel method of efficient preparation of foamed articles of polyvinyl chloride resins having high expansion ratios at the ambient pressure.
2. Description of the Prior Art
Various kinds of materials are utilized for plastic foamed articles. Polyvinyl chloride resins, among them, have excellent weatherability, can be colored readily, can be formed into various shapes and have other desirable properties. However, polyvinyl chloride resins are not utilized in large quantities like polyurethanes or polyolefins because articles of high expansion ratios are not easily prepared from polyvinyl chloride resins.
Foamed articles of high expansion ratios of polyvinyl chloride resins have generally been prepared under high pressure by introduction of an inert gas or by decomposition of foaming agents. In these conventional methods, an apparatus for the high pressure is necessary and productivity is kept low because of batch operations. Cost of the preparation in these methods is inevitably high.
When foaming at the ambient pressure is attempted, a problem arises that vinyl chloride resins have low melt viscosity above the temperatures of decomposition of conventional foaming agents to cause dissipation of the gas formed by the decomposition. When a foaming agent having a low decomposition temperature is utilized, another problem arises that melting of the polyvinyl chloride resin is insufficient, again causing dissipation of the gas formed by the decomposition. Thus, foamed articles of high expansion ratios can not be prepared by these conventional methods.
As another method of preparing foamed articles of polyvinyl chloride resins of high expansion ratios at the ambient pressure, a method of crosslinking a copolymer of vinyl chloride and ethylene by radio active irradiation, followed by decomposition of a foaming agent by heating was proposed (Japanese Patent Publication Showa 48-4863). In this method, cost of the foaming becomes high because expensive apparatus must be utilized for the irradiation. Furthermore, the polyvinyl resins tend to be aged and discolored by the irradiation.
A method of preparation of foamed articles of polyvinyl chloride resins of high expansion ratios having good cushoning property, weatherability and flame resistance by crosslinking and foaming of plastisol of polyvinyl chloride resins comprising a copolymer resin of vinyl chloride and a monomer having hydroxyl group, carboxyl group, epoxy group or alkoxy group, a plasticizer, a crosslinking agent and a foaming agent was disclosed (Japanese Patent Publication Showa 52-44588). This method has a problem that storage stability of the plastisol is inferior because crosslinking takes place gradually at the ambient temperature by the presence of the crosslinking agent and, thus, viscosity of the plastisol is increased with time. This method has also other problems that the expansion ratio can not be made to a high level as expected because the acid utilized as the crosslinking agent has tendency to suppress the foaming and that unreacted crosslinking agent remaining in the article is bleeded to the surface of the foamed article to cause various undesirable phenomena such as unsightly surface and removal of ink printed on the surface.